U.S. Pat. No. 3,833,408 to Matthies, herein incorporated by reference, describes the application of methyl alkyl siloxane compositions as lubricants for conductive information discs comprising a molded plastic disc having audio and video signal information in the form of geometric variations in a spiral groove. These discs are coated first with a conductive material which acts as a first electrode of a capacitor, then with a dielectric layer and a final layer of lubricant. A metallized stylus acts as a second electrode of the capacitor. The information signals are monitored by the stylus which notes changes in capacitance between the stylus and the disc surface as the information signals, in the form of a surface relief pattern, pass beneath the stylus.
Further developments in this system have produced a disc which is made of a conductive plastic material, e.g., a polyvinylchloride homopolymer or copolymer resin containing sufficient amounts of conductive carbon particles so that the disc can provide capacitance readout. The plastic resin at the surface of the disc surrounds the carbon particles to produce a dielectric surface layer. This development has eliminated the need for separate coatings of metal and a dielectric layer on the surface of the disc.
The stylus, formerly made of metallized sapphire, has also been improved so that metallized diamond can be used. Diamond is a harder, longer wearing material than sapphire but also requires improved lubrication of the disc surface.
High density information discs are also being developed which do not require a conductive surface or a grooved surface, the stylus being maintained in synchronization with the information pattern track by means of electrical signals rather than by the groove walls.
These changes in the materials used for the high density information discs and the stylus have changed the requirements for the lubricant system and improved lubricants were required. Wang et al, in copending application Ser. No. 065,065 filed Aug. 9, 1979, now U.S. Pat. No. 4,275,101, which is a continuation of Ser. No. 937,819 filed Aug. 29, 1978, have described an improved lubricant system which comprises a fractionated, purified methyl alkyl siloxane of the formula ##STR1## wherein R.sub.1 and R.sub.2 are alkyl groups of 4-20 carbon atoms, x is an integer of 2-4 and y is an integer of 0-2 and wherein the sum of x and y is 4 or less. These lubricants have improved long term stability and resistance to temperature and relative humidity changes in the atmosphere.
High density information discs of the above type are subject to a phenomenon called carrier distress. Since the polyvinylchloride composition from which the discs are made is a heavily filled, heavily lubricated and heavily plasticized composition, degradation products that are produced during molding and on storage from reactions of the disc materials and excess, incompatible additives, bleed to the surface of the disc, forming a thin layer of organic and inorganic materials. This layer interferes with playback by collecting in the grooves and by building up on the stylus. The result can be locked grooves, or dropouts of information as the disc is played. This problem has been somewhat alleviated by cleaning the discs after molding and prior to lubrication with aqueous solutions which remove at least some of the surface layer. However, with time, additional materials bleed to the surface of the disc. This bleedout can be accelerated by exposure of the disc to high temperatures, on the order of about 100.degree. F., and high relative humidity, e.g., 90 percent and above. Lubrication of the disc has heretofore had little or no effect on reducing carrier distress. However, it would be highly desirable to be able to reduce carrier distress by means of a permanent layer on the disc surface.